1. Field of the Invention
The invention relates to a measuring apparatus for the determination of parameters of medical fluids, the apparatus having a measuring chamber device for receiving a disposable cassette with at least one measuring chamber through which the fluid to be measured is guided, and at least one measuring element for measuring a measured value whose value depends on the extent of the measuring chamber and allows for determination of the parameter, or of a value being in a fixed relationship thereto, with a known value for the extent of the measuring chamber. The present invention also relates to a method for the determination of medical fluids by leading the fluid through a measuring chamber, and measuring a measured value whose value depends on the extent of the measuring chamber in a measuring direction, the measured value allowing for determination of the parameter, or of a value being in a fixed relationship thereto, with a known value for the extent of the measuring chamber. Further included in the present invention is a method for the calibration of a measuring apparatus for the determination of parameters of medical fluids.
2. Description of the Related Art
It is, for example, helpful in dialysis treatment for the blood temperature, the blood volume and/or the air portion to be determined in the extra-corporeal blood circulation during the treatment.
For this purpose, for example, the blood is led during the dialysis treatment in the extra-corporeal blood circulation through a disposable cassette such as is described in DE 198 37 667 A1. Such a disposable cassette consists of a plastic part which, in turn, consists of two further parts. Passages and volumes are let into a hard part. This hard part is closed by a flexible film for the covering of the channels and volumes. The disposable cassette is introduced into a special receiving chamber, e.g. on a dialyser.
This chamber can, for example, be opened with the help of a pivotable door. The disposable cassette is inserted into the chamber, with the flexible film lying opposite a corresponding opposing piece on the machine such that the cassette can be operated with the aid of actuators and sensors on the machine side. The door is closed and thereby serves as an abutment. Sensors, e.g. a temperature sensor or a pressure sensor, can be provided in the chamber for the determination of various parameters of the blood during the dialysis treatment.
Furthermore, a light barrier can be provided to determine whether blood is flowing through the passages. A run time measurement can be carried out through a measuring chamber in the disposable cassette with the aid of an ultrasonic measuring unit, which allows a determination of the blood density, of the concentration and/or of the haematocrit value.
To be able to determine the desired parameters precisely from the run time, the ultrasonic path must be known as accurately as possible. With the known solution of DE 198 37 667 A1, the disposable cassette is for this reason held in the receiving chamber with the aid of spacer bolts and screw connections such that a precise spacing results.
Such a design is, however, complex and requires a fairly long preparation of the dialysis treatment. The arrangement is sensitive to contamination and wear due to the spacer bolts. The pressing forces which act on the cassette during a treatment, which can take several hours, can result, for example, in a plastic deformation of the cassette and thus of the spacing. On the other hand, openings through the cassette and the film are required to be able to fix the spacer bolts in place.
Starting from this prior art, it is the object of the present invention to provide an apparatus and a method with which the accurate determination of a parameter of a medical fluid is possible whose value is dependent on the extent of a measuring chamber, wherein the apparatus should be insensitive to wear and contamination and the handling of the apparatus and of the measuring process should be easy.
This object, as well as others, is solved by a measuring apparatus and a measuring method according to the present invention. A calibration process in accordance with the present invention for a measuring apparatus in accordance with the invention is also provided.
With the measuring apparatus in accordance with the invention, at least one distance sensor unit is provided which is arranged relative to the measuring chamber such that the distance measured changes relative to the extent of the measuring chamber. With such a distance sensor unit, a distance can be determined, which depends on the extent of the measuring chamber, during the measurement of a measured value which depends on the extent of the measuring chamber. The measured value allows the determination of the medical parameter in question when the extent of the measuring chamber in the measuring direction is known. It is therefore possible with the apparatus in accordance with the invention to determine not only the measured value dependent on the distance during the treatment, but also the distance, or its change, itself. The desired parameter can thus be directly determined from these values and e.g. represented and monitored on a corresponding display unit. Naturally, both the parameter determined in this manner and other values in a fixed relationship with it can be used directly for the control of the treatment, e.g. for a change of the temperature, the flow rate or other parameters relevant during dialysis treatment.
At the start of the treatment, e.g. of a dialysis treatment, only the disposable cassette has to be connected and closed. No particular care must be taken to precisely adjust the distance of the two elements of the receiving chamber for the disposable cassette, since the signal of the distance sensor unit is taken up during the treatment and the extent of the measuring chamber in the measuring direction of the measuring element can be accurately determined.
Tolerance fluctuations of the disposable receiver during the treatment, e.g. due to different fluid pressure values or temperature changes, are automatically taken into account and do not require any special correction.
The distance sensor unit can here be disposed directly in the region of the measuring chamber. The extent of the measuring chamber in the measuring direction can be determined directly in this manner without the measured distance signal having to be additionally converted into the extent of the measuring chamber. In other arrangements, it can be advantageous for the distance sensor unit to be located offset with respect to the measuring chamber. The distance between the two elements can likewise be determined, but the measurement of the fluid sent through the measuring apparatus remains unaffected.
With an advantageous embodiment, the distance sensor unit comprises two parts which do not contact one another and which are arranged relative to the measuring chamber such that their relative distance changes with the extent of the measuring chamber.
The distance sensor unit can be e.g. a capacitive unit, with the two parts comprising the electrodes of a capacitor. A change in the distance of the capacitor plates changes the capacity, which can be determined in a known manner. An optical distance measurement can likewise be provided.
With a particularly advantageous embodiment, the distance sensor unit comprises at least one unit for the generation of an electrical and/or magnetic field and at least one field sensor for the measurement of the field or its change. If the distance between the field generation device and the field sensor changes, then the field also changes, e.g. in its strength and/or direction. A conclusion can be made on the distance change from this field change. With a suitable calibration, the distance can also be determined absolutely.
If a magnetic field is used for the distance measurement, the change of the distance can be determined e.g. by induced eddy currents.
However, the magnetic field measurement is particularly simple to realise with the aid of a magneto-resistive sensor such as is described in DE 197 46 199. With such a magneto-resistive sensor, the resistance of a sensor arrangement changes in an easily measurable manner when the magnetic field passing through it changes. If now an element generating a magnetic field, for simplicity""s sake e.g. a permanent magnet, is located e.g. on one side of the disposable cassette, and if a magneto-resistive sensor is located on the other side of the disposable cassette, then the signal of the magneto-resistive sensor is dependent on the distance which is in turn directly related to the extent of the measuring chamber. The electrical signal of the magneto-resistive sensor can be measured easily and evaluated precisely. With a sensor in accordance with DE 197 46 199, the direction of the magnetic field with respect to the sensor axis is detected with the help of a sensor. The arrangement in accordance with the invention is then selected such that the direction of the magnetic field at the location of the field sensor is unambiguously related to the distance of the field sensor from the field generation device in order to thus determine the distance from the direction of the magnetic field.
With the dialysis treatment, the accurate knowledge of the water content and/or of the blood density related to this is necessary. The density measurement can comprise e.g. a light absorption measurement. However, the measurement of the run time of an ultrasonic wave through the blood, with the sound velocity being dependent on the density, is particularly accurate. A conclusion can thus also be drawn on the haemocrit value from the run time. For the ultrasonic measurement, an ultrasonic sensor can be provided on one side of the measuring chamber and an ultrasonic receiver on the opposite side of the measuring chamber. Naturally, the sensor and the receiver can also be provided on one side of the measuring chamber and only an ultrasonic reflector on the other side.
With such a design of the measuring apparatus, the run-time of the ultrasonic wave can be defined as the measured value. The desired medical parameter can be determined from this measured value with a known distance of the ultrasonic measuring elements, with the determination of this distance or its change being accurately possible with the measuring apparatus in accordance with the invention. Naturally, the ultrasonic speed can also be itself determined in this manner.
The distance can be monitored at a point of the disposable cassette with the help of a distance sensor unit and thus a conclusion can be drawn on a change of the extent of the measuring chamber in the measuring direction. A higher accuracy can be achieved if distance sensor units are provided at a plurality of points of the measuring apparatus offset with respect to one another. In this manner, a tilting or displacement of the different parts of the measuring apparatus can also be detected.
With a method in accordance with the invention for the determination of parameters of medical fluids, a value being in a fixed relationship to the extent of the measuring chamber in the measuring direction or to the change in the extent is determined during the determination of a measured value whose value depends on the extent of a measuring chamber. With the method in accordance with the invention, a magnetic field is particularly advantageously determined for the determination of the distance of a field sensor and of a field generation device in order to be able to determine the extent of the measuring chamber from this distance. As described, a direction determination of the magnetic field can e.g. be used for this purpose when the direction of the magnetic field at the location of the field sensor depends on the distance of the field sensor from the field generation device.
An accurate monitoring of the extent of a measuring chamber, e.g. in a disposable cassette, during the dialysis treatment is possible with the measuring apparatus in accordance with the invention and with the measuring method in accordance with the invention. Parameters which are determined from a measured value which is dependent on the extent of the measuring chamber can thus be determined accurately and simply. The measuring apparatus is insensitive to contamination and wear and can be easily operated. A particularly accurate adjustment or separate monitoring of the distance is not necessary.